Down syndrome (DS) is the most common neurogenetic syndrome associated with intellectual disability, occurring in approximately 1:691 live births (Parker et al., 2010). Recent approaches to treatment in DS and other neurogenetic disorders have begun to focus on the earliest part of the lifespan. This type of targeted early treatment work will require highly sensitive and psychometrically sound outcome measures to demonstrate effects on early developmental trajectories in DS. However, young children with DS show phenotype-specific delays in motor development and early communication, which could confound the reliability and validity of early developmental assessments in the critical cognitive domain of executive function (EF). Therefore, rigorous psychometric validation of EF assessments is needed to advance treatment science for young children with DS. In this study, 210 young children with DS (between 2.5 and 7.99 years) will be assessed at four waves of data collection to establish reliability, validity, internal consistency, and sensitivity to change in the proposed battery of assessments. A promising set of laboratory assessments has been selected to measure various aspects of EF. The proposed measures will minimize phenotype-related confounds that threaten the reliability and/or the validity of early cognitive assessment in DS. Advanced quantitative analytic approaches utilizing latent variable models within a structural equation modeling framework will be used to investigate the psychometric properties of the proposed measures. Project outcomes will strengthen treatment science for young children with DS and contribute to the critical effort to improve developmental outcomes in this population.